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AlgebraHardSlope as a rate with units

Algebra practice question

A tank holds 60 litres of water and drains completely in 5 minutes at a constant rate. Which function gives the volume V, in litres, remaining t minutes after the drain is opened, for 0 ≤ t ≤ 5?

  1. A. V(t) = 60 − 12tCorrect
  2. B. V(t) = 60 − 5t
  3. C. V(t) = 12t
  4. D. V(t) = 60 − t/12

Answer: A. V(t) = 60 − 12t

Rate: 60 litres over 5 minutes = 12 litres per minute. V = 60 − 12t.

Why the other answers are wrong

B. V(t) = 60 − 5t
−5t uses the number of minutes as the rate.
C. V(t) = 12t
12t is the amount drained, not what remains.
D. V(t) = 60 − t/12
t/12 would take an hour to drain 5 litres.

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